This invention relates to an atomizer that creates liquid droplets through application of an electrical field.
Many processes depend on the formation of liquid droplets of controllable size. Examples of this include internal combustion engines, ink jet and bubble jet printers. Performance of most combustion engines depends strongly on how well the liquid fuel is injected into the combustion chamber or inside the carburetion system. The process of combustion is limited by the size distribution of fuel droplets sprayed into the air stream. The purpose of spray atomization is to create a very small size distribution of droplets with high surface area for heat and mass transfer. Typically, heat and mass transfer scale as d−2 (d is the droplet diameter) while the aerodynamic response time of the droplets scales as d2. Thus, the smaller the droplets, the more rapidly they evaporate while they are given more time for evaporation within the air flow stream.
While light fuels like octane have low vapor pressure and evaporate fairly rapidly, heavy hydrocarbons such as diesel and JP8 will take more heat and longer time to completely vaporize in the combustion chamber. It is therefore common to produce a much finer mist using high-pressure atomizers in diesel engines. The injection pressure delivered by plunger pumps to the spray nozzles in a diesel engine usually range from 1,500 to 7,000 psi. At these pressures, the droplets range in size from 10 to 100 μm with a Sauter mean diameter of approximately 50 μm.
In spark ignition engines, the issue of broad size distribution in the droplets causes less of a problem than in compression ignition engines. In compression ignition engines, the fine droplets burn too fast, and the larger droplets don't follow the flow path, leading to unburned hydrocarbons emissions or the formation of deposits in the engine.
However, uniform droplet size distribution has been difficult to achieve using conventional high-pressure spray atomizers. The broadening of the droplet size distribution can be attributed to several different processes. The atomization process, which is characterized as highly chaotic at the onset of jet breakup, results in different break-up wavelengths and therefore different droplet diameters. Further, after each droplet is formed and is being issued into the flow stream, small satellite droplets form in its tail. These two mechanisms are inherently inter-dependent in that the wavelength of the column of liquid injected out of a nozzle (or what forms immediately after the liquid jet leaves a nozzle) dictate the shape of the main droplets and number of trailing droplets. Some of the droplets tend to coalesce after injection to form larger droplets. The rate of collision and coalescence is a function of the turbulence intensity in the flow stream, the initial droplet size distribution and the number density of the droplets.
In some jet engines, fuel and air are mixed by dispersing the fuel into a high velocity stream of air, where the air turbulence provides the energy for atomization (so-called air-blast mixing). This approach suffers from the drawbacks that (1) uniform droplets are not created and (2) atomization depends on the air velocity, which can vary.
Recent efforts have focused on the electrostatic dispersion of the fuel droplets to reduce coalescence. However, the potential payoff for focusing on this mechanism is extremely low. Researchers have been very interested in the process of jet instability and liquid column breakup. It turns out that the mode of growth of instability waves tend to lock in on external excitations, overriding the natural frequency of the fastest growing waves. For example, an acoustic force is commonly used for controlling jet breakup and atomization. However, acoustics cannot have direct impact on satellite droplet formation. A significant amount of research has also been vested in suppression of satellite droplets, mainly in the ink jet and bubble jet printer industries. The concept of “tail cutting” has been explored and demonstrated in microinjectors using a recently developed thermal ink jet atomizer. Using diesel fuel, approximately 30 μm droplets have been issued out of a 30 μm nozzle.
A method by which droplets of controllable size can be produced using low energies and pressures would be desirable.
In other applications, it is desirable to be able to dispense small volumes of fluids in the form of small droplets. Conventional methods of atomizing fluids do not provide the fine control needed to atomize small quantities of fluids efficiently. This leads to poor results and waste of the fluids.